1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for indicating a precoding matrix for use in downlink data.
2. Discussion of the Related Art
In a multiple antenna system or a Multiple-Input Multiple-Output (MIMO) system, precoding may provide high peak-to-average system throughput by providing a beamforming gain and a diversity gain to a transmitting side and a receiving side. However, a precoding scheme should be appropriately designed in consideration of antenna setup, channel environments, system structure, etc.
Generally, a MIMO system which performs precoding in order to minimize complexity and control signaling overhead uses a codebook based precoding scheme. A codebook includes a number of precoding vectors/matrices predetermined between a transmitting side and a receiving side according to a transmission rank and the number of antennas. The receiving side estimates the state of a channel between the transmitting side and the receiving side using a reference signal transmitted by the transmitting side and transmits channel state information to the transmitting side. The transmitting side selects a specific precoding vector/matrix within the codebook according to the channel state information received from the receiving side to perform precoding on a transmission signal. The transmitting side then transmits the precoded transmission signal to the receiving side.
FIG. 1 is a diagram explaining a basic concept of codebook based precoding.
According to a codebook based precoding scheme, a transmitting side and a receiving side share codebook information including a predetermined number of precoding matrices according to a transmission rank and the number of antennas. The receiving side may measure a channel state through a received signal and may feed information about preferred precoding matrices based on the codebook information back to the transmitting side. Although FIG. 1 shows transmission of the information about the preferred precoding matrices by the receiving side to the transmitting side in units of codewords, the present invention is not limited thereto. When the receiving side transmits the information about the preferred precoding matrices to the transmitting side, precoding matrices other than optimal precoding matrices may be used for precoding of data transmitted by the transmitting side to the receiving side. However, such case is advantageous in that feedback overhead is reduced.
Upon reception of the feedback information from the receiving side, the transmitting side may select a specific precoding matrix from a codebook based on the received information. Then the transmitting side performs precoding by multiplying the selected precoding matrix by layer signals corresponding in number to the transmission rank. The transmitting side may transmit the precoded transmission signal to the receiving side through a plurality of antennas. Upon receiving the precoded transmission signal from the transmitting side, the receiving side performs an inverse process of precoding to restore a reception signal. Generally, the precoding matrix satisfies the condition of a unitary matrix U, such as U*UH=I. The inverse process of precoding may be performed by multiplying a Hermitian PH of a precoding matrix P used for precoding of the transmitting side by the reception signal.
Meanwhile, with advances in technology, the number of antennas which can be included in the transmitter has increased. However, if the number of transmission antennas increases, the size of a codebook increases and thus the amount of data which should be fed back also increases. Further, reception capabilities vary according to codebook design scheme. Therefore, the codebook should be designed so as to exhibit superior reception capabilities at the receiving side. In addition, a proper feedback scheme is needed which can reduce the amount of data fed back to the transmitting side.